Ordinarie mice, Mus musculus, and Natals are two different spieces and can not produce any hybride offspring. I don´t even think they are sexually interested in each other. What they can do is to be good companions and live peacefully together.
My sorce (Alwyn on mus.ifokus.se) says that the only problem with introduction is when/if the Natal/Natals are older. It´s best to introduce them to mice before 7 weeks of age.